My bet is that the OT special effects really couldn't achieve really strange body designs and that GL wanted to keep the PT pretty consistent with the OT, so he decided not to offer any really weird body plans -- although there are a few. Sebulba, a Dug, walks on his "arms" and uses his legs as "arms." There are also some "insectoid" species and Hutts are slug-like and Grievous can walk on two legs and also on all six -- though that's because of his robotics. Ithorians (hammerheads) and Talcs have a very non-human, non-earth creature sort of head design.

I do wish that GL had allowed for more body plans in the PT. It is strange for instance that there are no species in the 6 films that have the superior digitigrade walking/running style (a knee that bends backward like an Ostrich's instead of forward like ours), and the EU has a number of species with digitigrade locomotion and other stranger body designs.

I'm not sure you could really have an advanced species without cephalization, though -- or at least I'm having trouble imagining it. Any advanced creature on earth has a head of some sort, even cephelapods. Their heads are just in the middle between their appendages and their guts.

Some of the beasts deviate a little more from the normal body plan: acklays, some of the deep sea creatures in TPM, sarlaac, dianoga (the trash compacter monster), etc.

A lot of species share one common ancestor, the natural Humans which likely evolved from the Zhell race on ancient Coruscant, from where they settled the galaxy by slower-than-light and dimensional drive generation and sleeper ships before modern hyperdrive. Mirials, Zabraks, Arkanians, Ratattaki and Chiss all originate from the normal Humans, their genetic codes just changed a bit over time.

Others like Twi'Lek, Togruta, Faust (a lot like Humans) along Duros, Rakata, Selkath, Bothans (very humanoid) are unrelated to the Humans. In the fictional Star Wars universe, the "theory of evolution", which is true in this space fantasy world, seems to explain this by pure insanely unlikey coincidence. Otherwise, ancient races like the Celestials could have interferred with their evolution or maybe the Force is guiding the course of life as "lifeforms", which can be partially true in the real world, too.

Besides, only most species shown to the audience are humanoid, while there are 20 million sentient species at all in the Star Wars galaxy (we have seen a mere 5000 so far). Quite a possibility is, that most of them (99%) are non-humanoid and so not part of the greater galactic civilization as they can't interfere with most other species due to their serious differences in physical form and psychical abilities. So, humanoid species are the elites create civilizations build starships because they can and posess interest, while most others are non-humanoids living simple, just a bit better than non-sentient Terran mammals and are happy too.

As most members of the audience are humanoid elites too, Star Wars material focuses on them while nobody cares for the others except exceptions like Ikrit and Ooroo (I can't imagine Celegians have a techno-civilization like Humans or Duros while Kushiban live mostly like Terran wild cats) which are integrated into the Republic civilization.

For the real world, I believe in "intelligent design" as we, the Terran Humans, along our space-time continuum (the universe) with it's habitats (planets and moons) have been designed by transcendal transsapient beings from a higher plane of existence/different state of consciousness for the sake of existing or whatever. Together with Humans the likely sentient dolphings inhabit Earth, on other worlds there is a lot of sentient life too, many humanoid, most of them not. Who knows.

It's mostly about the costumes, animatronics are really hard for Star Wars 1977 back then. The remote controlled robots were already faulty easily, that's why they used guys in suits. That's why later, they did more advanced things like space slugs and Jabba the Hutt.